Optically active lactic acid has been produced by fermentation using a sugar as the substrate, but such fermentation processes require a considerable long period of time and troublesome operations for the separation, purification, etc. of the lactic acid formed, the concentration of the accumulated product being as low as the level of 10% at best. The production of D-lactic acid by fermentation is disclosed in, e.g. Japanese Laid-Open Patent Application No. 173596/88 and Japanese Patent Publication No. 38593/93.
For the enzymatic production of lactic acid, mention may be made of the production of an optically active .alpha.-substituted organic acid from a nitrile as the starting material (Japanese Laid-Open Patent Application Nos. 84198/90 and 224496/91) and the production of optically active lactic acid (Japanese Laid-Open Patent Application Nos. 99497/92 and 219987/93).
As other known processes, mention may be made of the production of an optically active -hydroxycarboxylic acid from an .alpha.-hydroxymethyl ester as the starting material (Japanese Patent Publication No. 63397/88), the production of an optically active .alpha.-hydroxcarboxylic acid (Japanese Laid-Open Patent Application No. 156892/90), the production of optically active lactic acid from 2-halogenopropionic acid (Japanese Laid-Open Patent Application No. 31690/84), the production of D-lactic acid from 1,2-propanediol (Japanese Laid-Open Patent Application No. 271787/92).
As processes using an amide as the starting material, mention may be made of the production of optically active .alpha.-oxyacid by a microorganism (Japanese Laid-Open Patent Application No. 88894/86), the production of optically active .alpha.-oxyacid (Japanese Laid-Open Patent Application No. 55098/87), the production of an optically active .alpha.-substituted organic acid (Japanese Laid-Open Patent Application No. 84198/90) and the production of an optically active .alpha.-substituted organic acid (Japanese Laid-Open Patent Application No. 224496/91).
Japanese Laid-Open Patent Application Nos. 88894/86, 84198/90 and 224496/91 suggests a possibility of converting a racemic amide into the corresponding optical active carboxylic acid in a general description without providing any specific example for direct production of optically active lactic acid from DL-lactamide, and it is not clear whether optically active lactic acid can be produced directly from DL-lactamide. Japanese Laid-Open Patent Application No. 88894/86 merely discloses a process in which only 2 kinds of microorganisms are used for the conversion of a specific .alpha.-hydroxyamide into the corresponding optically active .alpha.-hydroxycarboxylic acid. Japanese Laid-Open Patent Application Nos. 84198/90 and 224496/91 are directed to processes for converting an .alpha.-substituted nitrile or .alpha.-substituted amide into the corresponding .alpha.-substituted acid by a microbial enzyme reaction. However, this prior art does not contain any description of the production of D-.alpha.-substituted acid, such as lactic acid having a combination of hydroxyl group and low molecular weight alkyl group because all the products exemplified for the production of an .alpha.substituted acid from an -substituted amide are those having halogen, aryl group, aryloxy group, heterocyclic group, etc., bound to the optically active carbon, with the majority of the exemplified processes being related to another process for producing an optically active .alpha.-substituted acid from an .alpha.-substituted nitrile.
Japanese Laid-Open Patent Application No. 55098/87 discloses a process for producing L-lactic acid from DL-lactamide, but there is still not known any process for producing a predominant amount of D-lactic acid from DL-lactamide.
In addition, none of the above prior art refers to the efficient utilization of an enzyme and microorganism in each reaction.
Further, in the above prior art there does not appear any particular description with respect to the concentration of the optically active substance accumulated, the concentration if any being only a few % or less in a few examples.
The efficient utilization of an enzyme and microorganism and the accumulation of a high concentration of D-lactic acid and L-lactamide therefore have been an important task for the industrial production of both the compounds.